1. Field of the Invention
The present invention relates to a recording medium imaging device and an image forming apparatus capable of determining the kind of a recording medium.
2. Description of the Related Art
In a conventional image forming apparatus, the kind of a recording medium (i.e., size, thickness, and the like) has been set by a user through a computer as an external apparatus, for example, or through an operation panel provided on the main body of the image forming apparatus. Transfer conditions (such as a transfer voltage and the conveyance speed of a recoding medium in transfer) and fixing conditions (such as a fixing temperature and the conveyance speed of a recoding medium in fixing) in a transfer unit, for example, are controlled according to the setting.
In order to reduce the burden of the user in setting the kind of a recording medium through such a computer or an operation panel, there has been provided an image forming apparatus including a sensor for automatically determining the kind of a recording medium. The image forming apparatus including the sensor automatically determines the kind of a recording medium and then sets the transfer conditions and the fixing conditions according to the determination results.
More specifically, as discussed in Japanese Patent Application Laid-Open Nos. 2002-182518 and 2004-038879, some image forming apparatus determine the kind of a recording medium in such a manner that a CMOS sensor captures the surface image of the recording medium and detects the surface smoothness thereof from the captured image. The CMOS sensor directly captures a shadow cast by the unevenness of the surface, which enables the accurate determination of the recording medium. In distinguishing among glossy paper, plain paper, and rough paper, for example, the image forming apparatus can accurately determine the kind of a recording medium by detecting the presence of unevenness, or size and depth thereof.
In such prior art, however, paper dust can be generated on the recording medium in conveying the recording medium in the image forming apparatus. Dust can adhere to the recording medium or the recording medium can be scratched. If there are dirt or scratches due to such paper dust on the recording medium, a surface image with a characteristic different from an actual recording medium can be captured under the influence of the foreign matters. If the recording medium is determined based on the surface image containing such foreign matters, a determination accuracy for the recording medium decreases.